The R-T-B based sintered magnet (R represents a rare earth element, T represents one or more elements of iron group with Fe being an essential element, and B represents boron), a representative of which is Nd—Fe—B based sintered magnet, is advantageous for miniaturization and high efficiency of the machines used due to high saturation magnetic flux density, and thus can be used in a voice coil motor of a hard disk drive, etc. In recent years, the R-T-B based sintered magnet has been applicable in various industrial motors, driving motors of the hybrid vehicles, or the like. From the viewpoint of energy saving, it is desirable that the R-T-B based sintered magnet can be further popularized in these fields. However, when applied in the hybrid vehicles and the like, the R-T-B based sintered magnet will be exposed to a high temperature, and thus suppression on demagnetization at high temperature caused by heat becomes important. In the suppression on demagnetization at high temperature, a method of sufficiently improving the coercivity of the R-T-B based sintered magnet at a room temperature is well known as effective.
For example, as a method for improving the coercivity of the Nd—Fe—B based sintered magnet at a room temperature, a method in which part of Nd of the compound Nd2Fe14B which acts as the main phase is replaced with heavy rare earth elements such as Dy and Tb is well known. By replacing part of Nd with the heavy rare earth elements, the magneto-crystalline anisotropy is increased, and as a result, the coercivity of the Nd—Fe—B based sintered magnet at a room temperature can be sufficiently improved. In addition to the replacement with heavy rare earth elements, addition of elements such as Cu is also effective in improving the coercivity at a room temperature (Patent Document 1). By addition of element Cu, the element Cu forms, e.g., Nd—Cu liquid phase in the grain boundary, and thus the grain boundary is smoothened, inhibiting the occurrence of the reverse magnetic domains.
It is pointed out that, in order to improve the coercivity of the rare earth based magnet, inhibition on the movement of the magnetic domain wall of the occurred reverse magnetic domain is important, too. For example, Patent Document 2 has disclosed a technique in which fine magnetically hardening products of a non-magnetic phase are formed in the grains of the main phase R2T14B, and thus magnetic domain wall pinning is performed, thereby improving the coercivity. Moreover, Patent Document 3 has disclosed a technique for improving the coercivity by forming a magnetically modulated portion in the main-phase crystal grains, based on the same technical idea as Reference 2.